Teams are working to contain the damage from an oil spill in the Yellowstone River near Billings, Montana, caused by a ruptured ExxonMobil pipeline running under the river, officials said.
ExxonMobil is sending clean-up crews to Laurel, Mont. to mop up the thick band of oil on the banks of the Yellowstone River.
"We've shut down the pipeline and the segment where the release occurred has been isolated. And, obviously, we've been working in concert with all appropriate state and federal authorities on this," ExxonMobil Pipeline spokesman Kevin Allexon said.
"Obviously we are very, very, regretful that this has happened and we are working hard, in collaboration with all the local authorities to make sure we mitigate the issue," he said.
Allexon said the company doesn't know what caused the pipe to break.
"That's obviously subject of an on-going investigation at this point," he said.
The breach in the 12-inch diameter pipeline released an undetermined amount of oil into the river before workers were able to shut it down, according to Allexon.
Crews are now putting booms into place to prevent the leaked oil from spreading further.
Oil has been washing along shore from Laurel to Pompeys Pillar, prompting temporary evacuations Saturday morning.
Brent Peters, the fire chief for the city of Laurel, about 12 miles east of Billings, said the break in the pipe occurred late Friday night about a mile south of Laurel, according to The Associated Press.
Peters said due to concerns about possible explosions, and the overpowering fumes, about 140 people were evacuated in his district starting at about 12:15 a.m. Saturday.
They were allowed to return at about 4 a.m. after instruments showed fumes had decreased, according to the AP.
While the cause of the break is not known, Peters told the AP that speculation is focused on the possibility that the high water levels in the Yellowstone may have gouged out the river bed and exposed the oil pipe, which then may have been hit by debris.
"I haven't seen it this high for at least 15 years," Peters said.
The area has undergone widespread flooding in recent weeks due to a record amount of rainfall and a melting snowpacks in the mountains.
ABC News Radio and The Associated Press contributed to this report.